The invention concerns an automatic controlled transmission.
Automatic controlled transmissions are being increasingly used in motor vehicles. They can be transmissions with continuously variable ratio or multi-step transmissions shiftable under load or with traction interruption. To make those transmissions more comfortable when starting up and changing the ratio to lower ratio steps, a hydrodynamic converter is manually provided. On account of the low efficiency of the converter, therewith is provided a lock-up clutch which, in operation ranges, is engaged when the converter is no longer needed. In an engaged state, the lock-up clutch transmits the power from an input shaft of the converter to an output shaft.
The lock-up clutch is also used to damp out drive train vibrations that appear when driving through or during operation of critical rotational speeds. The clutch is then slightly opened and adjusted to a slip value of up to 3% depending on rotational speed, the load, and torsional vibrations.
The slip of the lock-up clutch causes an increase in oil temperature dependent on the differential rotational speed and requires an accordingly increased flow rate of the coolant through the converter. In known transmissions, a sufficient flow rate of coolant, is ensured by the friction surfaces of the lock-up clutch being grooved or providing a nozzle system in the piston of the clutch. Although the increased flow rate of coolant in the slip operation has a favorable effect on the temperature of the lock-up clutch and of the converter, the steps already known reduce the transmitting capacity of the lock-up clutch closed. In part, this disadvantage can be compensated by an elevated differential pressure on the piston of the lock-up clutch by increasing the pressure in the converter. Also the loss of pressure on the lock-up clutch has to be compensated by the high flow rate of the coolant. Narrow limits are, of course, set on the pressure increase, especially in the case of heavy engines, since the structural parts cannot be dimensioned as large as desired.
The problem, on which the invention is based, is to improve the cooling of a lock-up clutch of a converter in the slip operation without impairing the transmitting capacity of the lock-up clutch in engaged state.